The air fuel ratio of the mixture for an internal combustion engine is often controlled in dependence on the load with the aim of improving the emission property of the engine. The air fuel ratio can be adjusted by using an electronically controlled fuel injection system or by controlling the fuel jet of a carburetor with a solenoid valve.
The electronically controlled fuel injection system requires electricity from the time of start up, and has the disadvantage of requiring a control unit which is bulky and expensive. When the fuel jet is controlled by using a solenoid valve, substantially less cost is required as compared to the electronically controlled fuel injection system, but the flow rate of fuel is required to be controlled at a high precision. Because the flow rate of fuel is extremely small, it is highly difficult to achieve a desired level of precision.
A proposal has been made to address this problem without using an electronic controller by providing a carburetor that can adjust the air fuel ratio with a mechanical arrangement. According to this proposal, the carburetor is provided with a first and second air passage that are communicated with an air bleed chamber for feeding air to the fuel passage (nozzle), and a cutaway is formed in the throttle shaft so that the second air passage is communicated with the air bleed chamber via this cutaway. The cutaway is configured such that the cross sectional area of the second air passage is narrowed, and the air fuel ratio is reduced owing to the reduction in the supply of air to the air bleed chamber when the throttle opening is great (when the engine load is great). See JP2004-137928A, for instance.
However, in the carburetor proposed in this patent document, the length of the flow passage from the inlet of the second air passage to the air bleed chamber is so great that a significant time delay is inevitable from the time the second air passage is narrowed until the time the air fuel ratio is actually changed. Furthermore, because of the need to form a cutaway in the throttle shaft, the diameter of the throttle shaft is required to be increased in view of ensuring an adequate cross sectional area for the second air passage. For the given size of the carburetor, increasing the diameter of the throttle shaft results in the reduction in the cross sectional area of the throttle valve with the result that the engine output property is adversely affected. Therefore, the cutaway has to be increased in size in view of ensuring an adequate cross sectional area of the second air passage so that the freedom in the configuration and positioning of the cutaway is impaired. Therefore, it is highly difficult to achieve both an adequate cross section area of the second air passage and a favorable response property of the engine at the same time.